The hypothesis that will be tested in this proposal is that proteins that regulate IgH enhancer activity also regulate early B cell differentiation. The specific aims are: 1. To correlate a decrease in Id transcription, and increases in E2-5 and Oct-2 activity, with the onset of IgH enhancer activity in differentiating B cells. (Oct-2 is a B-- cell-specific transcription factor that activates both the IgH enhancer and VH-region promoters.) 2. To block the initiation of B cell differentiation by overexpressing Id, antisense E2-5, and antisense Oct-2, individually and in combinations, thereby testing whether a decrease in Id, or increases in E2-5 or Oct-2, are necessary for the initiation of B cell differentiation. 3. To induce B cell differentiation by overexpressing E2-5, Oct-2, and antisense Id, individually and in combinations, thereby testing whether a decrease in Id, or increases in E2-5 or Oct-2, are sufficient for the initiation of B cell differentiation. Two in vitro model systems for B cell differentiation will be used: coculture of LyD9 cells with bone marrow stromal cells and Whitlock-Witte bone marrow culture. Electroporation and retroviral transfer will be used to introduce genes into cells.